Ad hoc networks are communication networks devoid of fixed infrastructure. A certain number of wireless stations are equipped with radio emission and/or reception means and with appropriate protocols to form the nodes of the ad hoc network.
These stations making up the ad hoc network can be in the form of fixed or portable computers, pocket computers, mobile telephones, vehicles, electrodomestic appliances, etc. The emission-reception means can also be associated with simple objects such as sensors or actuators. An ad hoc network of sensors thus makes it possible to perform information collection for example with a view to monitoring or controlling installations.
The success of ad hoc networks depends a great deal on the lifetime of the stations constituting the nodes of the network. Energy saving is a crucial factor for designing long-lifetime sensor networks, in particular because the nodes are generally powered by cells which are generally expensive and difficult, or even impossible, to replace or recharge.
The access protocols for a conventional transmission medium (for example IEEE 802.11), require that the radio receivers of the stations be turned on permanently, always ready to receive the signal. This “ready to receive” mode consumes a great deal of energy. But if there is no transmission on the channel, the energy is wasted by this passive listening (or “idle listening”). This problem is particularly crucial in sensor networks with light traffic of the ad hoc type where the channel is free most of the time.
In order to solve this problem, procedures exist which make it possible to reduce the cost overhead of passive listening. The expression “passive listening” is understood to denote energy-consuming but fruitless active listening to a radio channel by a receiver node, that is to say listening without receiving any signal intended for this receiver node during listening.
Figuring among the known procedures are those according to which a receiver node listens to the radio channel intermittently. The emitted signals then generally comprise a preamble followed by a data frame. Such procedures are called preamble sampling techniques.
FIG. 1 shows in a schematic manner, along the time axis t, a signal 111 which is emitted on the radio channel by an emitter node E destined for a receiver node R according to such a preamble sampling protocol. The signal 111 comprises a preamble 115, for example a frame which contains repetitions of known bit patterns, and a data frame 113.
From the outset it will be noted that the “active radio listening mode” of a node corresponds to the operation of the node when its radio reception means are activated, consume energy and are therefore able to receive a signal, if any, transmitted to the node, while the “inactive radio listening mode” corresponds to the operation of the node when its radio reception means are on standby and do not consume any energy.
The receiver node R, like each receiver node of the network, is in active radio listening mode (radio turned on) for brief and periodic waking moments, of determined duration, represented by the squares 129 along the time axis t. The time separating the start of two consecutive wakeup moments 129 is equal to T′w. These wakeup moments 129 are spaced out by long inter-waking periods 127, during which the receiver node is in inactive radio listening mode (radio turned off): no energy is then consumed for a radio listening task. The wakeup moments of the receiver nodes are not necessarily concomitant.
During the wakeup moments 129, the receiver node R switches to active radio listening mode for the moment so as to listen to the channel and determine whether there is a signal transmitted on the channel.
If the receiver node R determines that the channel is free, it returns to an inactive radio listening state (radio turned off) at the conclusion of the moment 129. On the other hand, if it detects, at the listening moment 129, the presence of at least one determined pattern of bits, it deduces therefrom the presence of a preamble on the channel and it remains in active radio listening mode until it receives the data frame 113 which follows the preamble 115 (period 117), if appropriate beyond the listening moment 129.
After receiving the data 113, the receiver node R returns to an inactive radio listening state.
Thus, in protocols of this kind, a node spends most of its time in inactive radio listening mode so as to reduce the passive listening and therefore save energy.
Additionally, when a node wishes to dispatch a data frame, it first listens to the channel. If it determines that the channel is occupied, it continues to listen until the channel is freed. On the other hand, if it determines that the channel is free, it dispatches a preamble, then the data frame. The duration of the preamble must be at least equal to T′w, to ensure that all the potential receiver nodes switch to active listening mode during the emission of the preamble and are thus able to receive the data which follow the preamble.
Thus when a node detects a preamble, to receive the data frame, it must listen to the channel continuously until the data frame is received.
Additionally, the WOR or Wake-on-Radio procedure (Chipcon AS, CC1100/CC2500 Wake on Radio Application Note (Rev 1.0) July 2005), is known, according to which the receiver also listens to the radio channel intermittently, in a manner similar to the receiver R of the procedure described above.
According to the WOR procedure, when an emitter node E wishes to transmit a data frame 123, it emits a succession of copies of the data frame prior to this data frame.
The WOR procedure, in a first so-called “no acknowledgement” mode, consists of the dispatching by an emitter node E′ of n successive copies of the data frame, which is spread over a period greater than the period T″w separating the starts of two successive wakeup moments.
In a second so-called “with acknowledgement” mode, the emitter node E′ stops dispatching the data frames as soon as it has received an acknowledgement dispatched by the receiver node R′ for which the data frame was intended.
Thus in this second mode, the emitter node E′ dispatches over the radio channel a maximum of n prior copies 1231, 1232, . . . , 123n of the data frame, n being a predefined number (for example, in the case considered, n is equal to five). After dispatching a data frame 123i, the emitter node E′ listens to the radio channel during a time span TACK so as to detect any acknowledgement signal that might be dispatched by the recipient of the data frame. If it does not detect any acknowledgement signal at the termination of this time span TACK, it dispatches a new copy of the data frame and so and so forth until it has dispatched five copies of the data frame. On the other hand, if the emitter node E′ receives an acknowledgement signal, it halts the emission of the copies of the data frame.
In the particular case described in FIG. 2, the emitter node E′ has dispatched three first copies of the data frame 1231, 1232 and 1233. During the channel listening following the emission of the third copy of the data frame, the emitter node E′ detects the transmission on the channel of an acknowledgement signal 114 dispatched by the data frame recipient node R′. It therefore does not dispatch any fourth and fifth copies of the data frame.
Receiver node side, the receiver node R′, is in active radio listening mode (radio turned on) during waking moments represented by the squares 139 along the time axis t, which are spaced out by long inter-waking periods 137. The duration separating the starts of two successive wakeup moments is equal to Tw2 seconds, during which the receiver node R′ is in inactive radio listening mode.
Every Tw2 seconds, the receiver node R′ wakes up to check whether there is a signal transmitted on the channel. For this purpose, the node switches to active radio listening mode during the period 139 to listen to the channel. If the node determines that the channel is free, it returns to an inactive radio listening state (radio turned off). On the other hand, if it detects, during the listening period 139, that the channel is occupied, it performs the necessary operations (detection of header bits indicating a data frame, detection of the synchronization bits, checking of non corruption of frame) to receive in its entirety and correctly a data frame transmitted on the radio channel. After correct receipt of the data frame, the receiver node R′ dispatches an acknowledgement signal, then returns to an inactive radio listening state.
With reference to FIG. 2, the receiver node R′ has not been able to correctly receive the first copy 1231 of the data frame dispatched by the emitter node E′, since it was not in active radio listening mode at the start of the transmission of this copy. The receiver node R′ is in inactive listening mode during the transmission of the second copy of the data frame 1232. On the other hand, it detects during a wakeup moment 139 the start of the transmission of the third copy 1233 of the data frame and remains in active listening mode until the end of the receipt of this third copy. Thereafter, it switches to emission mode to emit on the channel an acknowledgement signal 114 intended for the emitter node E′.
The advantage of this second mode of the WOR procedure is that the dispatching of an acknowledgement signal guarantees proper receipt of the data frame dispatched. In the event of non acknowledgement, the emitter node E′ can decide to dispatch the data frame again.
The duration of a wakeup moment 139 is fixed at least equal to the time which separates the start of transmission of a copy of a data frame i and the start of transmission of the next copy i+1, i.e. equal to the sum of the transmission time of a frame and of the time TACK in the course of which the emitter waits for the acknowledgement signal.
The lighter the traffic, the greater will be the passive listening of the receiver node during these wakeup moments, because the time TACK is taken into account in the duration of the listening period 139.
A need therefore exists, in particular in the case of the emission of a succession of copies of a data frame before the emission of this data frame, to allow a decrease in the heavily energy-consuming passive listening of the receiver node, while allowing the dispatching by the latter of a signal acknowledging proper transmission of the data frame.